linux/kernel/rcu/srcutiny.c
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   1// SPDX-License-Identifier: GPL-2.0+
   2/*
   3 * Sleepable Read-Copy Update mechanism for mutual exclusion,
   4 *      tiny version for non-preemptible single-CPU use.
   5 *
   6 * Copyright (C) IBM Corporation, 2017
   7 *
   8 * Author: Paul McKenney <paulmck@linux.ibm.com>
   9 */
  10
  11#include <linux/export.h>
  12#include <linux/mutex.h>
  13#include <linux/preempt.h>
  14#include <linux/rcupdate_wait.h>
  15#include <linux/sched.h>
  16#include <linux/delay.h>
  17#include <linux/srcu.h>
  18
  19#include <linux/rcu_node_tree.h>
  20#include "rcu_segcblist.h"
  21#include "rcu.h"
  22
  23int rcu_scheduler_active __read_mostly;
  24static LIST_HEAD(srcu_boot_list);
  25static bool srcu_init_done;
  26
  27static int init_srcu_struct_fields(struct srcu_struct *ssp)
  28{
  29        ssp->srcu_lock_nesting[0] = 0;
  30        ssp->srcu_lock_nesting[1] = 0;
  31        init_swait_queue_head(&ssp->srcu_wq);
  32        ssp->srcu_cb_head = NULL;
  33        ssp->srcu_cb_tail = &ssp->srcu_cb_head;
  34        ssp->srcu_gp_running = false;
  35        ssp->srcu_gp_waiting = false;
  36        ssp->srcu_idx = 0;
  37        ssp->srcu_idx_max = 0;
  38        INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
  39        INIT_LIST_HEAD(&ssp->srcu_work.entry);
  40        return 0;
  41}
  42
  43#ifdef CONFIG_DEBUG_LOCK_ALLOC
  44
  45int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
  46                       struct lock_class_key *key)
  47{
  48        /* Don't re-initialize a lock while it is held. */
  49        debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
  50        lockdep_init_map(&ssp->dep_map, name, key, 0);
  51        return init_srcu_struct_fields(ssp);
  52}
  53EXPORT_SYMBOL_GPL(__init_srcu_struct);
  54
  55#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  56
  57/*
  58 * init_srcu_struct - initialize a sleep-RCU structure
  59 * @ssp: structure to initialize.
  60 *
  61 * Must invoke this on a given srcu_struct before passing that srcu_struct
  62 * to any other function.  Each srcu_struct represents a separate domain
  63 * of SRCU protection.
  64 */
  65int init_srcu_struct(struct srcu_struct *ssp)
  66{
  67        return init_srcu_struct_fields(ssp);
  68}
  69EXPORT_SYMBOL_GPL(init_srcu_struct);
  70
  71#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  72
  73/*
  74 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
  75 * @ssp: structure to clean up.
  76 *
  77 * Must invoke this after you are finished using a given srcu_struct that
  78 * was initialized via init_srcu_struct(), else you leak memory.
  79 */
  80void cleanup_srcu_struct(struct srcu_struct *ssp)
  81{
  82        WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
  83        flush_work(&ssp->srcu_work);
  84        WARN_ON(ssp->srcu_gp_running);
  85        WARN_ON(ssp->srcu_gp_waiting);
  86        WARN_ON(ssp->srcu_cb_head);
  87        WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
  88        WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
  89        WARN_ON(ssp->srcu_idx & 0x1);
  90}
  91EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
  92
  93/*
  94 * Removes the count for the old reader from the appropriate element of
  95 * the srcu_struct.
  96 */
  97void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
  98{
  99        int newval = ssp->srcu_lock_nesting[idx] - 1;
 100
 101        WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
 102        if (!newval && READ_ONCE(ssp->srcu_gp_waiting))
 103                swake_up_one(&ssp->srcu_wq);
 104}
 105EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 106
 107/*
 108 * Workqueue handler to drive one grace period and invoke any callbacks
 109 * that become ready as a result.  Single-CPU and !PREEMPTION operation
 110 * means that we get away with murder on synchronization.  ;-)
 111 */
 112void srcu_drive_gp(struct work_struct *wp)
 113{
 114        int idx;
 115        struct rcu_head *lh;
 116        struct rcu_head *rhp;
 117        struct srcu_struct *ssp;
 118
 119        ssp = container_of(wp, struct srcu_struct, srcu_work);
 120        if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
 121                return; /* Already running or nothing to do. */
 122
 123        /* Remove recently arrived callbacks and wait for readers. */
 124        WRITE_ONCE(ssp->srcu_gp_running, true);
 125        local_irq_disable();
 126        lh = ssp->srcu_cb_head;
 127        ssp->srcu_cb_head = NULL;
 128        ssp->srcu_cb_tail = &ssp->srcu_cb_head;
 129        local_irq_enable();
 130        idx = (ssp->srcu_idx & 0x2) / 2;
 131        WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
 132        WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
 133        swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
 134        WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
 135        WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
 136
 137        /* Invoke the callbacks we removed above. */
 138        while (lh) {
 139                rhp = lh;
 140                lh = lh->next;
 141                local_bh_disable();
 142                rhp->func(rhp);
 143                local_bh_enable();
 144        }
 145
 146        /*
 147         * Enable rescheduling, and if there are more callbacks,
 148         * reschedule ourselves.  This can race with a call_srcu()
 149         * at interrupt level, but the ->srcu_gp_running checks will
 150         * straighten that out.
 151         */
 152        WRITE_ONCE(ssp->srcu_gp_running, false);
 153        if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
 154                schedule_work(&ssp->srcu_work);
 155}
 156EXPORT_SYMBOL_GPL(srcu_drive_gp);
 157
 158static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
 159{
 160        unsigned short cookie;
 161
 162        cookie = get_state_synchronize_srcu(ssp);
 163        if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
 164                return;
 165        WRITE_ONCE(ssp->srcu_idx_max, cookie);
 166        if (!READ_ONCE(ssp->srcu_gp_running)) {
 167                if (likely(srcu_init_done))
 168                        schedule_work(&ssp->srcu_work);
 169                else if (list_empty(&ssp->srcu_work.entry))
 170                        list_add(&ssp->srcu_work.entry, &srcu_boot_list);
 171        }
 172}
 173
 174/*
 175 * Enqueue an SRCU callback on the specified srcu_struct structure,
 176 * initiating grace-period processing if it is not already running.
 177 */
 178void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 179               rcu_callback_t func)
 180{
 181        unsigned long flags;
 182
 183        rhp->func = func;
 184        rhp->next = NULL;
 185        local_irq_save(flags);
 186        *ssp->srcu_cb_tail = rhp;
 187        ssp->srcu_cb_tail = &rhp->next;
 188        local_irq_restore(flags);
 189        srcu_gp_start_if_needed(ssp);
 190}
 191EXPORT_SYMBOL_GPL(call_srcu);
 192
 193/*
 194 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
 195 */
 196void synchronize_srcu(struct srcu_struct *ssp)
 197{
 198        struct rcu_synchronize rs;
 199
 200        init_rcu_head_on_stack(&rs.head);
 201        init_completion(&rs.completion);
 202        call_srcu(ssp, &rs.head, wakeme_after_rcu);
 203        wait_for_completion(&rs.completion);
 204        destroy_rcu_head_on_stack(&rs.head);
 205}
 206EXPORT_SYMBOL_GPL(synchronize_srcu);
 207
 208/*
 209 * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
 210 */
 211unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
 212{
 213        unsigned long ret;
 214
 215        barrier();
 216        ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
 217        barrier();
 218        return ret & USHRT_MAX;
 219}
 220EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
 221
 222/*
 223 * start_poll_synchronize_srcu - Provide cookie and start grace period
 224 *
 225 * The difference between this and get_state_synchronize_srcu() is that
 226 * this function ensures that the poll_state_synchronize_srcu() will
 227 * eventually return the value true.
 228 */
 229unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
 230{
 231        unsigned long ret = get_state_synchronize_srcu(ssp);
 232
 233        srcu_gp_start_if_needed(ssp);
 234        return ret;
 235}
 236EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
 237
 238/*
 239 * poll_state_synchronize_srcu - Has cookie's grace period ended?
 240 */
 241bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
 242{
 243        bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
 244
 245        barrier();
 246        return ret;
 247}
 248EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
 249
 250/* Lockdep diagnostics.  */
 251void __init rcu_scheduler_starting(void)
 252{
 253        rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
 254}
 255
 256/*
 257 * Queue work for srcu_struct structures with early boot callbacks.
 258 * The work won't actually execute until the workqueue initialization
 259 * phase that takes place after the scheduler starts.
 260 */
 261void __init srcu_init(void)
 262{
 263        struct srcu_struct *ssp;
 264
 265        srcu_init_done = true;
 266        while (!list_empty(&srcu_boot_list)) {
 267                ssp = list_first_entry(&srcu_boot_list,
 268                                      struct srcu_struct, srcu_work.entry);
 269                list_del_init(&ssp->srcu_work.entry);
 270                schedule_work(&ssp->srcu_work);
 271        }
 272}
 273